1. Technical Field
The present invention relates to passive infra-red security sensors.
2. Prior Art
Such sensors are used in intruder detection systems and rely on detecting fluctuations in the infra-red radiation falling on an infra-red sensitive detector which are caused by movement in the field of view of the detector. In passive infra-red sensors currently on the market, the field of view of the infra-red detector is divided into a plurality of discrete zones so that, as the intruder crosses the zones, the infra-red input to the detector fluctuates. This fluctuation above the ambient level of infra-red radiation received by the detector from the surroundings can be detected by suitable electronic circuitry. By appropiately tuning the frequency to which the electronic circuitry is sensitive, the presence of an intruder can be distinguished from long term, relatively slow fluctuations in the ambient infra-red radiation level.
In prior art passive infra-red sensors, the zones are defined by an optical arrangement such as a multi-faceted mirror. A passive infra-red sensor using such mirrors is described in U.S. Pat. No. 3,703,718. This reference also discloses the general type of electronic circuitry suitable for use with such passive infra-red sensors.
The optical system for defining the zones may also be defined by a segmented Fresnel lens. Such a sensor is disclosed in GB-A-No. 2 124 363. When the zones are defined by optical arrangements such as a segmented Fresnel lens or a multi-faceted mirror, the radiation is focused onto the detector. Thus the field of view of the detector in a plane parallel to the mounting plane of the detector will be relatively small close to the detector. An intruder crossing in front of and close to the unit causes a high frequency signal. The sensitive elements, having some thermal mass, cannot easily respond to higher frequency heat signals, and their sensitivity generally decreases at 20 dB/decade above approx. 0.5 Hz. Thus targets close to the unit generate a relatively small signal output because of their high frequency. This results in reduced sensitivity close to the sensor itself.
The size of the field of view is also important in determining the sensitivity of the sensor. If an intruder substantially fills the field of view as he passes across it, then nearly all of the radiation emanating from the intruder will be focused onto the detector giving a high probability of detection. The greater amount of radiation can compensate for the reduction in signal caused by its higher frequency. If, however, the field of view is so small that it can be filled by a rat or mouse then the presence of such an animal may also trigger the sensor effectively causing a false alarm. If the field of view only covers a small portion of the intruder then insufficient energy may be focused onto the detector to overcome the poor high frequency response of the detectors so an alarm condition will not be produced. Therefore, a sensor using focusing optical arrangements tends to have very poor sensitivity close to the sensor but good longer range characteristics.
It has also been proposed to provide very short range sensor designed for energy saving in which no optics at all are employed. Such a sensor uses a detector disposed behind a window which itself effectively defines a single large zone. Such a sensor is capable only of detecting movement close to the unit, typically within a range of about 3 meters since at any distance from the detector the zone becomes so large that as an intruder enters the zone it does not produce any significant change in the amount of infra-red radiation being received by the detector.
As well as single element detectors, passive infra-red sensors using multiple element detectors have been proposed. For example, dual element detectors are commonly used. Each element of the detector is itself a separate infra-red detector. The elements are referred to as positive and negative elements respectively in dependence on the sense of the output deviation for a given variation in the incident radiation. The optics are arranged to that the zones of each element are different so that an intruder will cause a variation in the infra-red radiation falling on one element relative to the other. The outputs from the elements are processed to produce a differential output. If the differential output exceeds a predetermined threshold an alarm signal is produced. In this way variations in the overall intensity of the ambient infra-red radiation falling on the detector are compensated.
With this type of detector it is possible for the zones of the respective elements to overlap so that at some plane remote from the detector the zones provide sheet coverage, so that the combined zones give a field of coverage in the shape of a beam with a large included angle, in excess of 90%, centred on the sensor. In this way any intruder passing through the vertical plane where sheet coverage is provided must be detected. In conventional passive infra-red sensors such sheet coverage can only be provided by distorting the optics. The provision of optical arrangements for such coverage can result in very expensive and complicated designs.